In the development of a petroleum lubricating oils the trend has been directed to more and more drastic refining methods to reduce the tendency of such oils to form carbon and/or sludge deposits. While such highly refined oils possess many advantages, their resistance to oxidation, particularly under severe operation conditions, is generally decreased; they are more prone to form acidic oxidation products which are corrosive, and which cause undesirable increases in the viscosity of the lubricant.
To overcome the tendency of such highly refined oils to form carbon and/or sludge deposits on various operating parts of the engine, such as pistons, rings, valves, etc., various oil-soluble metal-containing detergent compounds now well-known in the art, have been incorporated in lubricating oil compositions. Such metal-containing organic compounds, while effective as detergents for dispersing the precursors of deposits within the oil itself, rather than permitting them to form deposits on the engine parts, had the disadvantage of forming ash deposits in the engine. To overcome this disadvantage, so-called ashless detergents were developed, and are now well-known in the art.
The organic compounds, both the metal-containing and the ashless, while imparting detergency properties to the lubricating oil compositions containing the same do not inhibit the oxidation of such lubricating oil compositions at high temperature operating conditions.
It has been reported by K. J. Humpris and Gerald Scott (JCS Perkins II, pp. 826-830, 1973) that catechol phosphites unexpectedly react quite rapidly with cumene hydroperoxides. Such rapid reaction was attributed by the authors to the in situ formation of ion-radicals by the phosphites which catalyze the decomposition of the hydroperoxides. Further, the authors reported that cyclic phosphites as enhancers of hydroperoxide decomposition are more effective than simple esters of phosphorous acid.
We, therefore, set out to find an oil-soluble polyhydroxy compound which could be converted to a cyclic phosphite by a commercially feasible preparation. Our tetramers of a mixed alkyl phenol described in our U.S. Pat. No. 3,876,709 are soluble in lubricating oils and have spacially close pairs of hydroxy groups (benzene ring carbons are joined to a benzene ring carbon connecting ortho positions to form the tetramer) which might, if reaction conditions were found, lend themselves to formation of a cyclic phosphite. Such tetrameric alkylphenols are obtained from ring carbon joining of a 2,4-dialkylphenol and a p-alkylphenol and can be represented by the following formula: ##STR3## wherein R.sub.1, R.sub.2, and R.sub.3 can be the same or can be different alkyl hydrocarbon groups. Such tetramers are poly-hindered phenols because all positions ortho to the hydroxyls are substituted by either alkyl or aromatic moieties. Preferably R.sub.1 and R.sub.2 are the same size alkyl hydrocarbon groups of from one to twelve carbon atoms and R.sub.3 is a different alkyl hydrocarbon group whose carbon atom content makes the sum of carbons in the six R groups equal to from 44 to 72 or greater.